Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Potassium, symbol K (from Latin kalium, “alkali”), chemically reactive, extremely soft metallic element. In group 1 (or Ia) of the periodic table (see Periodic Law), potassium is one of the alkali metals. The atomic number of potassium is 19.


Potassium was discovered and named in 1807 by the British chemist Sir Humphry Davy. The metal is silvery white and can be cut with a knife. It has a hardness of 0.5. Potassium exists in three natural isotopic forms, with mass numbers 39, 40, and 41. Potassium-40 is radioactive and has a half-life of 1.28 billion years. The most abundant isotope is potassium-39. Several radioactive isotopes have been artificially prepared. Potassium melts at about 63° C (about 145° F), boils at about 760° C (about 1400° F), and has a specific gravity of 0.86; the atomic weight of potassium is 39.098.

Potassium metal is prepared by the electrolysis of fused potassium hydroxide or of a mixture of potassium chloride and potassium fluoride. The metal oxidizes as soon as it is exposed to air and reacts violently with water, yielding potassium hydroxide and hydrogen gas. Because hydrogen gas produced in the reaction with water burns spontaneously, potassium is always stored under a liquid such as kerosene, with which it does not react.

Potassium is found in nature in large quantities, ranking eighth in order of abundance of the elements in the crust of the earth, in various minerals such as carnallite, feldspar, saltpeter, greensand, and sylvite. Potassium is a constituent of all plant and animal tissue as well as a vital constituent of fertile soil.


Potassium metal is used in photoelectric cells. Potassium forms many compounds resembling corresponding sodium compounds, based on a valence of 1. A few of the element's most important compounds follow. Potassium bromide, a white solid formed by the reaction of potassium hydroxide and bromine, is used in photography, engraving, and lithography, and in medicine as a sedative. Potassium chromate, a yellow crystalline solid, and potassium bichromate, or potassium dichromate, a red crystalline solid, are powerful oxidizing agents used in matches and fireworks, in textile dyeing, and in leather tanning. Potassium iodide, a white crystalline compound that is very soluble in water, is used in photography for preparing gelatin emulsions and in medicine for the treatment of rheumatism and overactivity of the thyroid gland. Potassium nitrate, a white solid prepared by fractional crystallization of sodium nitrate and potassium chloride solutions, is used in matches, explosives, and fireworks, and in pickling meat. It occurs naturally as saltpeter. Potassium permanganate, a purple crystalline solid, is used as a disinfectant and germicide and as an oxidizing agent in many important chemical reactions. Potassium sulfate, a white crystalline solid, is an important potassium fertilizer and is also used in the preparation of potassium alum. Potassium hydrogen tartrate, commonly known as cream of tartar, is a white solid used in baking powder and in medicine.